1. Field of the Invention
The present invention relates to a process and apparatus for the production of a blank mold for glass fiber optical waveguides.
2. Description of the Prior Art
It has become standard to use blank molds in the manufacture of glass fiber optical waveguides. These molds have a core and a sheath, in which the core glass material is applied to the inner wall of a glass tube by chemical deposition from the vapor phase, the internally-coated glass tube is caused to collapse by temperature treatment, and this blank mold is finally drawn out to a glass fiber.
Internal coating processes of this type, also known as modified chemical vapor deposition (MCVD) processes have been customary for a long time. In these processes, a suitable heat source used in the coating or collapsing system is moved one or more times along the glass tube in a longitudinal direction, with the corresponding temperature treatment being carried out in only one direction as a rule. For coating, metal halides, e.g., pure or doped silicon halides, are converted at high temperatures to the corresponding oxides, with the chemical conversion and the deposition process occurring practically simultaneously.
The need for increasing the manufacturing speed in glass fiber manufacture and the resulting increased output have already been the reason for increasing the burner capacity to arrive at shorter collapse times. Because, according to the formula: ##EQU1## where v(r) is the collapse rate,
p.sub.a is the pressure on the outer surface,
p.sub.i is the pressure on the inner surface, is the surface tension,
r.sub.a is the external radius
r.sub.i is the internal radius, and is the melt viscosity (temperature in 3C),
the quartz glass runs together faster with increasing softening. Nevertheless, an increased output cannot be achieved in practice, because the product quality obtained leaves something to be desired. This is because an increase in the burner capacity, with the burner settings customary at the present time, leads to the fact that the tubes that are to be collapsed are warped, distorted, or twisted, and can therefore no longer be held in a stable manner in the rotation axis during the manufacturing process. The collapse process must be discontinued without having the possibility of exhausting the actual potential of the burner.